There are, of course, many different types of position related sensors, that is, sensors that sense the position of some physical object and provide information as to the location of that object. One such sensor is shown and described in pending U.S. patent application Ser. No. 09/793,218 entitled “PRECISION SENSOR FOR A HYDRAULIC CYLINDER” and which, in turn, is a continuation-in-part of U.S. Pat. No. 6,234,061, issued on May 22, 2001, entitled “PRECISION SENSOR FOR A HYDRAULIC CYLINDER” and which was based upon U.S. Provisional application 60/104,866 filed on Oct. 20, 1998 and the disclosure of all of the foregoing applications and issued U.S. Patent are hereby incorporated into this specification by reference.
As can be seen, particularly in U.S. Pat. No. 6,234,061, a position sensor is disclosed that utilizes a cable that is coupled to the object to be sensed. This cable is wound around a spool and, as the cable unwinds or winds about the spool, the movement of the spool allows the position sensor to accurately determine the rotational movement of the spool and translate that rotational movement into a precise location of the object. In principle, therefore, the position sensor causes the cable to wind about the spool in a plurality of successive coils when the object moves in the direction toward the coil and unwinds those coils as the object moves away from the spool.
Accordingly, one of the difficulties with the aforedescribed type of position sensor is to have some system or mechanical function that provides a easy means to direct the cable to wind and unwind about the spool in an orderly fashion so that the winding function creates an even coiling of the cable without overlapping of any of the individual windings that make up the aggregate cable coil. The overlapping of individual windings of the cable coil has a deleterious effect on the accuracy of the position sensor.
Accordingly, one means of mechanically coiling the cable about the spool is to provide a guide in the form of a frame member or other fixed structural member associated with the rotatable spool such that the cable passes through a hole in that guide so that the guide serves to locate the cable along the spool in winding the cable thereon. Typically the hole formed in the guide is a small, circular hole and the cable therefore passes through that small, circular hole in the guide to reach the spool when it is wound and unwound in a cable coil as the spool rotates.
The hole-type guide, however, feeds the cable from one location. As consecutive windings of the coil are laid down, the new windings get wound about the spool further and further away from the feed point, which is the location of the stationary hole in the guide. At some point during winding, the position of an individual winding of the cable becomes so remote from the stationary small, circular, stationary hole that the cable winds over itself. In this way, the winding cable type of position sensor may be limited in sensing distance by the number of consecutive winds that can be made without the overlapping of individual windings of the cable coil.
Accordingly, it would be advantageous to have a position sensor that can utilize the feature of a cable wound about a spool that is not so constrained by the problem of the cable winding over itself but can be more assured that the continued winding of the cable will properly position each winding of the cable about a spool adjacent the previous winding so that the position sensor can be capable of an extended range of position sensing.